This invention relates to the determination of the topography of three dimensional objects and, more particularly, to an improved apparatus and method of moire topography.
Moire topographical contouring techniques have become recognized as simple and powerful means of three-dimensional object shape analysis. Both shadow and projection moire methods have been successfully used for various metrological applications including contouring of large objects and living bodies, for obtaining difference in object shapes, and for deflection and vibration analyses. In shadow moire techniques, a single large grating is typically disposed in front of the object, and the illuminating and reflected beams both pass through this grating. In projection moire techniques, the projection beam passes through a first grating, and the deformed version of this grating on the object is imaged at a second grating.
The relative advantages of the two methods have been extensively discussed in the literature and the projection moire method has been shown to generally offer greater flexibility than the shadow moire method.
In order to implement moire topography as a practical tool, recent investigations have been directed towards processing fringe patterns with increasing degrees of automation. Generation and acquisition of fringe data for automated analysis have been attempted using electro-optic, electronic and video techniques. Extensive automated analysis of moire fringes with the aid of digital computers is rather recent, and continuing efforts are being made in this direction.
A typical automated fringe processing scheme involves first digitizing the fringe pattern from a photograph or from an on-line video signal. The digitized picture is then processed, such as on an interactive computer system to detect fringe peaks, assign fringe orders and then plot the results. Fringe peak detection algorithms generally depend on the interrogation of the intensity values of the picture and are thus greatly affected by undesirable noise in the pattern.
Moire patterns have inherent noise due to the original gratings, especially when coarse gratings are used. Topographical fringes, in addition, are often plagued by spurious moire in the regions of large slopes. It has long been recognized that in a shadow moire arrangement, these artifacts can be eliminated by a grating translation technique. (J. B. Allen and D. M. Meadows, Appl. Opt. 10(1), 210 (1971), and J. Wasowski, Opt. Commun. 2, 321, (1970)). It has also been analytically shown (e.g. by Allen et al.) how these unwanted patterns disappear by photographic integration.
It is an object of the present invention to improve the quality of the moire patterns obtained in projection moire systems.